1. Field of the Invention
This invention relates generally to a digital-to-analog convertor, and more particularly to an integrated circuit for performing a digital-to-analog conversion and a successive approximation analog-to-digital conversion in which the transfer characteristic is nonlinear.
2. Prior Art
Previous digital-to-analog (D/A) and analog-to-digital (A/D) converters having a nonlinear transfer characteristic require sophisticated and expensive circuitry to implement the desired transfer function. Because of the complexity of these converters, they cannot be contained in one integrated circuit package, but require several integrated circuit chips to accommodate all of the required circuitry to implement the desired transfer function. Furthermore, the majority of the known D/A converters and successive approximation A/D converters have monotinicit problems.
A nonmonotonic D/A converter is one in which the analog signal output thereof does not increment in the same direction with the successive application at an input thereof of binary words of increasing value. Of the known D/A converters, all require relatively closely matched components in order to be monotonic. More particularly, the components of the known D/A converters must be matched within one-half LSB in order for the converter to be monotonic. One LSB (least significant bit) is equal to the inverse of 2.sup.n, where n is equal to the number of bits in the binary word.
One well known and widely used D/A converter is commonly called an R-2R ladder D/A converter. This type of converter employs a resistor ladder network which contains twice as many resistors and as many switches as there are bits in the binary work which is being converted to an analog signal. This converter can also be used in a successive approximation A/D converter. The resistors of this network must be closely matched in order for the converter to be monotonic. More particularly, the resistor in the least significant bit branch of this network must be matched within one-half LSB of the termination resistor in order for the circuit to be monotonic. In a like manner, each branch of the network must be within one-half LSB of the total of the branches in parallel with and of lower order than that branch in order for the circuit to be monotonic. In an eight bit D/A converter, for example, the resistors must be matched within 0.2%.
Other D/A and successive approximation A/D techniques employ either capacitor networks or transformer networks. The components of these networks must also be closely matched in order for the converter to be monotonic. The majority of the known D/A converters which are monotonic require sophisticated and expensive processing techniques. Those D/A converters which are constructed of discrete components require the selection of relatively high accuracy components, such that the components are matched within one-half LSB. Therefore, these discrete circuits are also relatively expensive if they are monotonic.
The attractiveness of the R-2R D/A converter is that it can be designed such that actuation of any or all of the switches contained therein does not change the current through the branches of the resistor network. That is, as each switch is actuated, the current in each respective branch of the network will be supplied to a summing amplifier and will be equal to the current in that branch prior to the actuation of that switch. It can be appreciated that if a change in current resulted with the actuation of any one of the switches, an error would result which would be proportional to the current change. This error could be sufficient to cause the circuit to be nonmonotonic. Some of the other D/A converters do not have this inherent advantage and compensation must be made for the change in current which results with the actuation of the switches therein.
The problem of matching the value of components in a D/A converter circuit is further complicated by another factor. These circuits have been fabricated in the past by providing circuit connections between the components in the well known manner, with the type of connection depending upon the type of circuit, such as an integrated circuit or a discrete circuit. The contact points of these connections are ohmic contacts in that they may present some resistance to the circuit. The resistance of these ohmic contacts contributes to the mismatch between components and branches of the D/A converter. Since the amount of resistance presented by these contacts may vary considerably and cannot be predetermined, the mismatch which may result therefrom cannot be easily determined before the circuit is fabricated. Accordingly, the permissible mismatch between components and branches in these D/A converters must be less than one-half LSB to permit some tolerance for the resistance presented by ohmic contacts in the circuit. The resistance offered by these ohmic contacts may also contribute to the inaccuracy of those circuits in which a current change may occur as discussed above. The inaccuracy of such circuits may be the result of a nonlinear reponse of the circuit and/or an undesirable scaling of the gain of the circuit. Both of these factors can be present as a result of unpredictable ohmic contacts in the circuit. Accordingly, it can be appreciated that it is desirable to reduce the number of these ohmic contacts to a minimum.
In those integrated circuit D/A converters which depend upon current flow through various components and branches thereof, the geometry of the components must be comparable to the amount of current drawn thereby. Accordingly, these components may, in some cases, be quite large, thereby increasing the area of the integrated circuit chip which they are a part of. Also, the geometry of these components must be matched within the tolerances discussed above. It can be appreciated that it is desirable to maintain the area of an integrated circuit chip to a minimum and to obviate the necessity of matching the geometries of its components.